It stands for Standard Automotive Mfg. Co. Los Angeles California. They manufactured replacement manifolds for early flat head engines.
You will find that S on a lot of A and V8 ignition parts, like pong plates and coils...S was one of several suppliers of ignition stuff as original equipment. Bohnalite was the main supplier of aluminum manifolds and heads, and was a major contributor to a real revolution that we still use...180 degree manifolding. This stuff came through Ford production routes and so was not conspicuously marked.